1. Field of the Invention
This invention relates to a welding composition having and producing a high strength, high toughness, stress corrosion resistant steel weldment. More particular, this invention relates to a high strength, high toughness, stress corrosion resistant weldable steel filler metal that can be employed to weld high strength, high fracture toughness alloy steels and produce a satisfactory weldment that has exceptionally high ultimate tensile and yield strength in combination with both high toughness and high stress corrosion resistance that, together, make it desirable for aerospace vehicular and other fracture critical structures.
2. Description of the Prior Art
Design requirements for metallic materials used in aerodynamic structures or the like include a high strength to weight ratio, high fracture or notch toughness, high stress corrosion, and ease of fabrication. A stress corrosion resistance to fracture toughness ratio (K.sub.ISCC /K.sub.IC) greater than 0.5 is highly desirable for aircraft structural components as well as any application where the maximum operating load is two or less times the steady state sustained load. Such a ratio insures that no stress corrosion cracking will occur during sustained load operation if the structure is designed to resist brittle fracture at maximum operating load. The terms "K.sub.ISCC " and "K.sub.IC " are the stress intensities in units of KSI .sqroot.inch (KSI equal thousands of pounds per square inch) below which, respectively, stress corrosion cracking will not occur within 1000 hours in 3.5 percent by weight sodium chloride solution, and brittle fracture will not occur.
As referred to herein, fracture resistance is measured in terms of notch toughness (CVN, or Charpy V-Notch), a measure of resistance to fracture under impact loading expressed in foot pounds (FT-LBF) or Joules (J) in the presence of a notch. Fracture toughness (K.sub.IC) is resistance to fracture under loading in the presence of a crack. In the welding composition of this invention, notch toughness measured as Charpy V-Notch (CVN) can be closely correlated empirically with the fracture toughness measurement obtained by the fracture mechanics test for K.sub.IC. For example, a Charpy V-Notch value of 35 foot pounds absorbed energy is equivalent to 115 Ksi .sqroot.inch value of fracture toughness for the steels of this invention. K.sub.Q is an approximate fracture toughness value that is used because the steels of this invention are so tough that the specimens had insufficient thickness to obtain K.sub.IC data. Fracture resistance also is a function of stress corrosion resistance (K.sub.ISCC) which measures resistance to crack growth in a corrosive environment under sustained load in the presence of a crack.
In U.S. Pat. No. 4,076,525, entitled "High Strength Fracture Resistant Weldable Steels," issued Feb. 28, 1978, to Clayton B. Little and Paul M. Machmeier, the descriptive matter of that patent being incorporated by reference herein for details that are omitted herefrom, there is described a wide variety of steels that have been developed for a broad spectrum of usage, including the special needs for the aerospace industry. As noted therein, many of the steels such as the HY-180, 300M D6ac, maraging steels and others provide various combinations of strength, fracture toughness and stress corrosion resistance. Some may also be welded. For example, U.S. Pat. No. 3,502,462 discloses steels in the range of up to 197 Ksi maximum yield strength (tensile) having excellent toughness and stress corrosion resistance. Despite this, there has been a need, particularly in the aerospace field, for a steel that is at once weldable and provides the best combination of low weight with good toughness and stress corrosion resistance at higher strength levels than heretofore available. Particularly, the steel should have up to about 270 Ksi ultimate strength (TUS) or about 245 Ksi yield strength (TYS) with good fatigue endurance limits. The composition of such a steel, referred to AF 1410, was provided by U.S. Pat. No. 4,076,525. However, arc welding of these exceptionally high strength, high toughness steels, using known materials, was deemed to produce weldments tending towards reduced soundness, e.g. porosity; and reduced values of mechanical properties described hereinbefore. Development of a welding composition that would provide arc-produced weldments of acceptably high quality was therefore needed.
Expressed otherwise, none of the commercially available welding compositions, or alloy steel filler metals, would meet the quality weldment requirements that allowed higher strength structures to be designed with no weight or fabricability penalties and still obtain good fracture toughness and stress corrosion resistance. Specifically, the welding composition of this invention had to provide increased resistance to crack growth in corrosive environments when compared to the prior art structural alloy steel weld metal.
From the foregoing, it can be seen that the prior art was not totally satisfactory in providing a welding composition for welding the high strength high toughness, stress corrosion resistant steels required in the aerospace industry and the like.
Specifically, none of the prior art weld wire compositions, or steel filler metals would provide the desired quality arc-produced weldments having the requisite properties in the ranges of 200-230 Ksi yield strength, 210-250 Ksi ultimate strength, and CVN equal to at least 30 foot pounds. Moreover, the prior art weld wire compositions did not provide the even more difficult to obtain combined ranges of 200-230 Ksi yield strength, 210-240 Ksi ultimate strength and CVN values of at least 35 foot pounds; and did not provide a K.sub.ISCC greater than 60 .sqroot.inch and K.sub.IC of at least 115 Ksi .sqroot.inch.